Selenium compounds as brighteners in copper plating baths



United States Patent W 3,532,610 SELENIUM COMPOUNDS AS BRIGHTENERS IN COPPER PLATING BATHS Arthur H. Du Rose, Richmond Heights, Ohio, assignor to Kewanee Oil Company, Bryn Mawr, Pa., a corporation of Delaware N0 Drawing. Filed Oct. 27, 1967, Ser. No. 678,550 Int. Cl. C23b 5/18, 5/46 U.S. Cl. 204-52 2 Claims ABSTRACT OF THE DISCLOSURE Compounds of the formula wherein R is part of a ring structure with the two attached carbon atoms and together with said carbon atoms forms a ring selected from the group consisting of benzene, naphthalene, pyridine, quinoline, pyrimidine, pyridazine, pyrazine, thiophene and pyrrole have been found to act as brighteners in copper cyanide electroplating baths. The ring structures of these compounds may be further substituted with one or more of the following: lower alkyl, amino, bromo, chloro, acetyl, hydroxy, and nitro. Likewise elfective are ethers of such compounds, especially when the ring is benzene, and selenodiazolo substituted biphenyls. Further additives may be added to broaden the bright current density range.

This invention relates to the use of various selenium compounds in electroplating from copper cyanide baths, more particularly to a bath composition which is particularly adapted to produce bright coatings of soft, ductile electroplates of copper.

Numerous attempts to obtain such bright coatings have been made. For instance, it has been proposed to use alkali metal selenite as an addition agent to an electrolytic bath in relatively large amounts. Such baths had a number of disadvantages in that the selenites tended to break down with a resultant adverse effect on the bright plating range. During the electrolysis because of breakdown of the selenites, the anodes became blackened, forming insoluble compounds which were loosened from the anodes and tended to co-deposit with the plated metal, resulting in rough deposits which were commercially unuseable. Also in commercial operation, due to the resulting very narrow bright plating range, non-uniform appearance of the deposits resulted, and the deposits were in many cases insufficiently bright so that buffing was necessary.

Another teaching of the prior art may be found in US. Pat. 2,770,587 in which various selenium compounds having a valence of --2 are claimed as brighteners in alkaline copper plating. The use of various selenium compounds having a valence of 2 improved the brightness of the deposit over a wider current density range but in comparison with the instant invention the operable current density range is much less desirable. While the cited prior art patent relies on valency as the criterion for operability of the patented invention, it should be pointed out that valency in and of itself is a meaningless criteria unless the nature of the compound per se be considered. For example, selenium compounds and sulfur compounds are both known brightening additives for copper or nickel plating baths and are considered somewhat equivalent. T 0 illustrate, let us consider the effect of aromatic sulfates and sulfonates in nickel plating solutions. In both instances the valence of the sulfur is said to be +6 but the 3,532,610 Patented Oct. 6, 1970 sulfur in the sulfonates is easily reduced at the cahtodc to sulfide and effects a brightening of the deposit, while the sulfate is unaffected and has practically no effect.

The present invention is intended and adapted to overcome the difficulties and disadvantages inherent in prior electroplating baths of the type described. It is among the objects of the present invention to provide a bath composition in which a relatively small amount of addition agents is introduced resulting in clearly increased brightness with the plated surface, with a wider more uniform plating range.

It is also among the objects of the present invention to provide a bath composition which results in a plating which is brilliant, soft and ductile and which may be readily bufied if desired to cause the coating to flow, as for example, on steel.

It is further among the objects of the present invention to provide a bath in which there is no breakdown even after long use.

The brightening additives of the instant invention may be depicted by the following generic formula.

While the above compounds illustrate selenium as having a valence of 4 it should be noted that the compounds of the instant invention retain their aromatic character and may also be represented by the following resonant forms.

These selenium compounds of the instant invention can be made by reacting the appropriate ortho-diamino ring compound with selenous acid.

The compound of the instant invention is operable as a brightener in an alkaline copper plating bath at concentrations between about 0.001 and 0.5 gram-per liter. The preferred concentration range for the compound of the instant invention in the copper plating bath is between 0.001 and 0.02 gram per liter. In this preferred range there is very little visible difference in deposit appearance throughout the range which is advantageous in that the exact concentration is not highly critical.

The use of the additive of the instant invention produces a brightening eifect with all of the known alkaline copper plating baths. Use of the additives of the instant invention affect the brightening of copper deposits over a current density range as broad as -100 amps per sq. foot. Since alkaline copper plating baths are typified by the plating baths given in the examples that follow. These alkaline copper baths operate at temperature ranging from about 130 F. to about 185 F. and the current density can vary widely as can be discussed later in detail. In the following examples, all proportions are in grams per liter.

EXAMPLE 1 A basic solution of the following composition and conditions was prepared: CuCN 75 g./l. Free' KCN 18 g./l.

KOH g./l.

K CO 40 g./l.

Temperature, 155 F.

A.s.f.

Dull semi-bright 0-10 Semi-bright 10-60 Dull semi-bright 60-90 above 90 Mat To this solution 0.005 g./l. of dithioammelide was added with the following results:

A.s.f.

Very bright O- Very dull-semi-bright 30-40 Bright semi-bright -90 above 90 Mat When the dithioammelide was increased to 0.15 g./l. the results were:

A.s.f. Very bright 0-37 Dull 37-60 Semi-bright 60-100 Mat above 100 It should be noted that the dull intermediate current density band shown on Hull Cell panels is not so apparent when panels are plated at or near this CD in beakers or when parts are plated in production tanks.

EXAMPLE 2 To the same basic solution as used in Example 1, 0.005 g./l. of 1,2-selenodiazolo benzene was added and a Hull Cell panel plated as before:

A.s.f. Semi-bright '0-3 Bright 3-150 Dull above 150 When 0.005 g./l. of dithioammelide was added to this solution the low CD area 0-3 a.s.f. became bright.

4 EXAMPLE 3 To the same basic solution as used in Example 1, 0.015 g./l. of 1,2-selenodiazolo naphthalene was added. In this case the current density ranges were:

v A.s.f. Dull semi-bright 0-15 Bright 15-80 Mat above When 0.01 g./l. of thioammelide was added the current density ranges became:

A.s.f. Bright 0-90 Mat above EXAMPLE 4 A solution equivalent to that of Example 1 but as a sodium formulation was used; i.e. NaCN, NaOH and Na CO were used in place of the potassium salts. When 0.006 g./l. of 1,2-selenodiazolo benzene was added the de- Notice that this deposit is not as bright as the corresponding deposit in Example 1 and that the upper limiting CD is lower. This is generally true for sodium formulations as compared with potassium. To the above solu tion 0.005 g./l. of dithioammelide was added with the following results:

A.s.f.

Bright 0-50 Semi-bright 50-80 Mat above 80 When the dithioammelide was increased to 0.05 g./l. a

very dull CD band was obtained at 40-70 a.s.f., which shows that it is possible to use too much of the auxiliary sulfur type brightener even in conjunction with the selenoorganic brightener.

EXAMPLE 5 To the basic solution as used in Example 1, 0.002 g./l. of 1,2-selenodiazolo-3-amino benzene was added, with the following results:

A.s.f. Bright 0-100 Dull semi-bright -140 Mat above When the selenium compound was increased to 0.01 g./l. the bright area lost some lustre although it would still be desrcibed as bright, and the upper limiting CD was increased to about a.s.f.

EXAMPLE 6 As in Example 5 but using 0.002 g./l. of 2,3-se1enodiazolo pyridine H se A.s.f. Semi-bright -5 Bright -100 Semi-bright 100-120 Mat above 120 (2) 1,2-se1enodiazo1o-3,6-dibromo-benzene Br N (3) l,2-selenodiazo1o-4-ethy1 benzene (4) 1,2-selenodiazo1o-4,6-dimethy1 benzene CH3 N (5) 2,3-selenodiazo1o-phenol (6) 1,2-selenodiazolo-5-chloro benzene (7) 1,2-se1enodiazo1o-5-nitro benzene N Se (8) 2,3-selenodiazo1o-5-amino pyridine N NH 6 (9) 2-methy1-4,5-selenodiazo1o pyridine (10) 3,4-se1enodiazolo pyridine N :Se

3,4-selenodiazo1o-6-br0mo pyridazine =Se 4,5-selen0diazo1o-3-amino pyridazine Se=N 4,S-selenodiazolo-Z-methyl pyrimidine cnz Se N 5,6-se1enodiaz0lo pyrimidine 2,3-se1enodiazolo-5-amino pyrazine Se NHZ Ny N% 2,3-se1enodiazolo-6-bromo .pyrazine 1,2-se1enodiazo1o-5-amino naphthalene @ijN/se 2,3-selen0diazolol-benzazine I N N\Se 2,3-se1enodiazo1o thiophene fiL Se 3,4-selenodiazolo pyrrole N N Se (21) di-(3,4-selenodiazolo benzene) ether (22) 3,4,3',4-di(selenodiazolo) biphenyl N=Se As will be noted from the previoius examples, the addition of a small amount of dithioammelide improves the brightness in the extremely low current density areas. The use of dithioammelide and its equivalents are disclosed in detail in US. Pat. No. 2,862,861 and describe the operable dithioammelide or its equivalents as a thio substituted six member heterocyclic ring compound wherein the members of the ring comprise l-3 nitrogen atoms and the balance carbon atoms, each nitrogen atom forming a part of a separate azomethine group and being connected to two carbon atoms, at least one carbon atom of one 2120- methine group carrying a substituent of the class consisting of thiomercaptide and alkylthiol. Typical examples falling under this description are 2,4,6-trimercapto triazine, dithioammelide, thioammeline, 4,6-diamine-2-mercapto pyrimidine, 4,6-diamino-2-methylmercapto pyrimidine, 2,4- dimercapto pyrimidine, 4-amino-6-hydroxy-2-mercapto pyrimidine, 6-amino-4-hydroxy-2-methyl mercapto pyrimidine, 4-hydroxy-2-mercapto-6-methyl pyrimidine, and 2- mercapto pyridine. All of these compounds are useful in conjunction with the compound of the present invention to improve the brightness of copper electroplate in the extremely loW current density areas. However, care should be taken to hold the concentration of these heterocyclic compounds below 0.05 gram per liter if a dull electroplate is to be avoided in the medium current density areas. However, if all plating is done in the low current density range up to 10 grams per liter of these compounds can be used to atfect a fully bright plate in the extremely low current density areas. Preferably, however, the concentration of dithioammelide is held between 0.001 and 0.05 gram per liter.

A broad range of copper cyanide concentration may be employed with the brightening agents of the invention, and, in this respect, the copper cyanide concentration may range from about 45 to about 120 grams per liter. When no agitation is employed, best results are obtained when the concentration is from about 75 grams per liter to about 120 grams per liter. When the concentration falls below about 75 grams per liter it has been found that the brightness is adversely atfected in the absence of agitation In general, an increase in the copper cyanide concentration above about 120 grams per liter does not appreciably affect the brightness of the deposit obtained in non agitated solutions. On the other hand, when the plating solution is agitated, it has been found that the preferred copper cyanide concentration should be materially reduced for the obtainment of optimum results. Thus, when vigorous agitation is employed, a copper cyanide concentration ranging generally from about 55 to about 85 grams per liter has been found best. When the solution is agitated, therefore, it will be found that the brightness contributed by the disclosed compounds is affected unless the copper cyanide concentration is adjusted to compensate for the degree of agitation, or vice versa.

As is well known to those skilled in the art of copper cyanide plating, a complex potassium cuprous cyanide compound is formed between the potassium cyanide and copper cyanide. The actual formula of the complex varies according to the temperature conditions of the solution among other things. In this regard, the potassium cyanide concentration excluding the free potassium cyanide generally amounts to about 1.46 times the amount of copper cyanide employed. Thus, baths containing copper cyanide in amounts, between about 45 and grams of copper cyanide per liter would contain potassium cyanide excluding the free cyanide, of from about 65 to about 146 grams per liter to form the complex. The potassium cyanide concentration is thus based upon a potassium copper cyanide complex having the approximate formula K Cu(CN) It is necessary in copper cyanide solutions to have suflicient cyanide present to form a complex, otherwise, unstable conditions result. In this regard, there must be an excess of free cyanide such as sodium or potassium cyanide to insure that the complex is formed.

It has been found that generally greater quantities of free cyanide must be present when the solution is agitated than when the solution is not agitated. For example, when the solution is not agitated, it has been found that when the concentration of the free cyanide falls below about 4.0 grams of free potassium cyanide per liter, a dull deposit results. A similar dullness is detected above a free cyanide concentration of about 30 grams per liter. The best results in nonagitated solutions are obtained when the free cyanide concentration ranges from about 10 grams per liter to about 20 grams per liter. On the other hand, when vigorous agitation is employed as described heretofore, it has been found that best results are 0-btained when the free cyanide concentration ranges generally from about 14 to about 25 grams per liter. When the concentration of free cyanide exceeds about 25 grams per liter, the cathode efiiciency is decreased appreciably. It will be apparent because of the necessity for free cyanide that the maximum limiting quantity of potassium cyanide is about 171 grams per liter when the copper cyanide concentration is about 100 grams per liter. The 171 grams per liter of potassium cyanide being the sum of the potassium cyanide associated with the copper cyanide and the free cyanide.

In preparing the basic solution, copper cyanide is preferably added to an aqueous solution of potassium or sodium cyanide in the desired amounts, and potassium or sodium hydroxide thereafter added to obtain the desired operating pH range. In general, a wide range of pH may be tolerated in the solution with optimum results being obtained in the pH range between about 11.5 and 13.5 in the case where there is no agitation, and in a pH range between about 12.5 and 13.5 in the case where there is vigorous agitation. Good results may be obtained in either case, however, at pHs ranging from about 9-14.

To buifer the solution against changes in pH, and to act as a complexing agent for divalent copper ions and impurity ions, potassium citrate in amounts generally ranging from about 35 to about 75 grams per liter has been found suitable. Best results have been obtained when the potassium citrate is employed in amounts ranging from about 45 grams per liter to about 65 grams per liter. It will be apparent that it is not essential that potassium citrate be employed as a butter or, in fact, that any buffer be utilized in the plating solution. In some cases it is desirable that impurities such as trivalent chromium not be complexed and solubilized. Other buffers such as Rochelle salts EDTA, etc. may be employed also, and, in this regard, the Rochelle salts may be employed generally in amounts ranging from about 5 to about 55 grams per liter. Usually the Rochelle salts have been found to be more effective in the agitated solutions than in the nonagitated solution.

during the plating process optimum results in nonagitated solutions may be obtained over a somewhat wider temperature range than in the case where vigorous agitation is employed. For example, temperatures ranging from about 140 F. to about 180 F. may be employed without agitation of the solution, whereas, a temperature from about 150 F. to about 160 F. is preferably employed when there is vigorous agitation. Again, it will be apparent that the ranges set forth herein with respect to an agitated and nonagitated solution are principally illustrative of the invention, and operating conditions will vary so far as their optimum is concerned. Between the extreme case of agitation illustrated, and the other extreme where there is relatively no agitation, solution temperatures of from about 140 F. to about 185 F. may be employed. Although temperatures as low as 130 F. have been successfully employed, they are not strongly recommended since the brightness of the plate obtained tends to diminish if the temperature of the plating solution falls below about 140 F. in the case where agitation is not employed. On the other hand, when temperatures in excess of about 160 F. are employed without agitation slightly higher current densities should be utilized. The bright plating current density range tends to increase somewhat when higher temperatures are employed. It is pointed out, however, that temperatures as high as about 185 F. have been employed with success utilizing the brightening agent described herein.

Broadly, current densities up to about 150 amps, per square foot of cathode surface area may be utilized according to the invention. Interrupted current appears to permit a wider and somewhat higher range of current densities and is usually preferred since it aids in eliminating polarization, and further minimizes film deposits on the anode. Suitable cycles for the use of interrupted current may have an on time of up to about 90 seconds and an 011 time of from about to about 50% of the on time. A suitable cycle is 7 seconds on and 2 seconds otf. Periodic reverse current may be used also but in most cases has no beneficial action. With a continuous current, optimum results in the form of maximum brightness have been obtained in nonagitated solutions when the current density ranges from about 10 to about 35 amps, per square foot of cathode surface area, whereas, when vigorous agitation and a continuous current are employed, it

has been found that a preferred range of current densities from about 5 to amps per square foot is best, whereas, with an interrupted current, the range is broadened out and may be raised to from about 5 to amps per square foot.

I claim:

1. An electrolytic bath for the plating of bright copper comprising an aqueous alkaline solution of a cyanide of copper and an amount of 3,4-selenodiazolo pyridine sufficient to exert a brightening effect on the deposit.

2. An electrolytic bath for the plating of bright copper comprising an aqueous alkaline solution of a cyanide of copper, an amount of 3,4-solenodiazole pyridine sufficient to exert a brightening effect on the deposit, and a secondary brightener consisting of a thio-substituted six-member heterocyclic ring compound wherein the members of the ring comprise from one to three nitrogen atoms and the balance carbon atoms, each nitrogen atom forming a part of a separate azomethine group and being connected to two carbon atoms, at least one carbon atom of one azomethine group carrying a substituent of the class consisting of thiol, mercaptide and alkylthiol, said alkylthiol having from one to three carbon atoms, said thio-substituted compound being dissolved in said bath in an amount from about 0.001 gram per liter to about 0.05 gram per liter.

References Cited UNITED STATES PATENTS 2,732,336 1/1956 Ostrow 20452 2,770,587 11/1956 Ostrow 20452 2,814,590 11/1957 Portzer et a1 20452 2,861,929 11/1958 Martin et al 20452 2,862,861 12/1958 Moy 20452 2,873,234 2/1959 Passal 20452 2,881,121 4/1959 Foulke et a1. 20452 2,881,122 4/1959 Foulke et al 20452 2,955,992 10/1960 Laue 20452 3,030,282 4/1962 Passal 20452 3,186,926 6/1965 Hofmann et al. 20452 XR FOREIGN PATENTS 1,177,450 9/ 1964 Germany.

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 

